This application addresses the cellular basis of time coding in the barn owl's brainstem auditory system. Temporal information is processed in the nucleus magnocellularis, which projects to the nucleus laminaris (NL), where interaural time differences (ITDs) are first computed. Because precise temporal coding in the inputs to NL is critical to the detection of ITDs, one aim is to determine how phase-locking changes during circuit development. These studies will be carried out in parallel with studies of features associated with temporal coding such as the regulation of myelination and the expression of K+ channels and GluR subtypes. ITD coding neurons generally possess bipolar morphology in both birds and mammals, with inputs from each ear segregated into dendritic trees. Because this segregation improves ITD coding in modeling studies, the quality of ITD coding will be compared with cell morphology in chickens, owls and parakeets, to determine if species differences exist. The barn owl's sound localization is more accurate than a chicken's. A second aim is to determine if this increased accuracy is due to computational power (i.e., more neurons devoted to a particular ITD computation) and/or to specific adaptations that improve an individual neuron's ability to encode ITD, such as changes in number of inputs, improvements in phase-locking, changes in dendritic length, etc. Increased neurogenesis in the progenitor cell population is the primary reason for the hypertrophy of the owl NL. Owl circuit development follows the same pattern as the chicken until fairly late, then small developmental changes create a different phenotype and consequent large change in behavioral acuity. A third aim is to test the hypothesis that changes in the expression of cell adhesion components mediate the cell separation typical of owl NL. It is possible that changes in neurogenisis and cell adhesion may be sufficient to underlie evolution of hypertrophied owl circuit. The similarity in cell types between the cochlear nucleus angularis and the mammalian posteroventral cochlear nucleus supports homology between the two nuclei, and/or convergent evolution of cells specialized for encoding changes in sound level. The fourth aim is to test these hypotheses in a study that correlates NA cell morphology with response types.